Communication service providers are subject to very strict bandwidth usage spectrum constraints, such as technically mandated specifications and regulations imposed by the Federal Communications Commission (FCC), which currently requires that sideband spillage, namely the amount of energy spillover outside a licensed band of interest, be sharply attenuated (e.g., on the order of 60 dB). While such limitations are adequate for traditional forms of modulation such as frequency modulation (FM), they are difficult to achieve using more contemporary, digitally based modulation formats, such as M-ary modulation.
Keeping the sidebands attenuated sufficiently to meet industry or regulatory-based requirements using such modulation techniques mandates the use of very linear signal processing systems and components. Although linear components can be implemented at a reasonable cost at relatively low bandwidths (baseband) used in telephone networks, linearizing such components, especially RF power amplifiers, becomes a very costly exercise.
RF power amplifiers are inherently non-linear devices, and generate unwanted intermodulation distortion products (IMDs), which manifest themselves as spurious signals in the amplified RF output signal, separate and distinct from the RF input signal. This intermodulation distortion is also referred to as spectral regrowth, or spreading of a compact spectrum into spectral regions that do not appear in the RF input signal. The distortion introduced by an RF amplifier causes the phase and amplitude of its amplified output signal to depart from the respective phase and amplitude of the input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the input signal.
One brute force technique for linearizing an RF power amplifier is to build the amplifier as a large, high power device and then operate the amplifier at a low power level that is only a small percentage of its rated output power, where the RF amplifier's transfer function is relatively linear. An obvious drawback to this approach is its overkill penalty--high cost, large size and poor efficiency. Other proposals to account for RF amplifier degradation include coupling a `pre-processing` correction loop in the path of the amplifier's input signal, and/or coupling a `post-processing`, feed-forward correction loop with the amplifier's output signal.
The purpose of a preprocessing correction loop is to modify the RF amplifier's input signal path. Ideally the control signal causes the signal path adjustment mechanism to produce a signal control characteristic that has been predetermined to be equal and opposite to the distortion expected at the output of the RF amplifier. As a consequence, when subjected to the transfer function of the RF amplifier, it will optimally effectively cancel the amplifier's anticipated distortion behavior. The mechanism may be made adaptive by extracting the RF error signal component in the output of the RF amplifier and adjusting the control signal in accordance with the such extracted error behavior of the RF amplifier during real time operation, so as to effectively continuously reduce distortion in the amplifier's output.
A post-processing, feed-forward correction loop serves to extract the amount of RF error (distortion) present in the RF amplifier's output signal, amplify that extracted distortion signal to the proper level, and then reinject the inverse of the amplified RF error signal back into a downstream output path of the RF amplifier, so that (ideally) the amplifier distortion is effectively canceled. To extract the error, the output of the RF amplifier is combined in an RF cancellation combiner with the RF input signal (which is used as a reference), in order that all carrier components (which give rise to the intermodulation distortion products) are effectively canceled, leaving only the RF error.
In the past, mechanisms for reducing such RF carrier components have involved the use of analog phase and amplitude adjustment circuits, which attempt to align the phase and amplitude of the two RF signals by the use of differential amplifier and phase detector circuitry installed in one or both RF signal paths. In addition, pilot tone injection and measurement units installed in the amplifier output path, upstream and downstream of a feed-forward error correction and reinjection location have been used. Because these conventional parameter monitoring and adjustment subsystems are stand-alone and autonomously controlled components, they not only increase the complexity and expense of an overall amplifier architecture, but rely on parameters other than the actual RF signals of interest (e.g., a pilot tone outside the desired spectrum).